The patch does away with needles and syringes and could transform the battle against future pandemics by painlessly inoculating patients with vaccines that could be sent out in the post and self-administered in the home by somebody with no medical experience.
In the developing world, the skin patches could eliminate the need for the costly medical infrastructure of mass-vaccination campaigns, which require trained medical personnel to inject vaccines, and expensive storage equipment. Skin patches also bypass the hazards of dirty needles.
The skin patch is "armed" with an array of microscopic needles made of biodegradable plastic that painlessly scratch the surface of the skin and dissolve harmlessly without trace after delivering the vaccine safely inside the body.
Tests have shown that the patch works just as well and possibly even better than conventional vaccines injected into the body with needles and syringes. The skin patches are biodegradable and, unlike dirty needles, there is no risk of accidental skin pricks and cross-contamination.
Experiments on laboratory mice showed that the skin patches are even better than injectable vaccines at preventing flu infections because the vaccine enters the body via the surface of the skin, which is an important site of the immune system. Most flu jabs, by comparison, deliver the vaccine into the muscles, which may not be the best place for triggering the all-important initial immune reaction.
Experts believe that, if immunisations against flu and other infectious diseases can be delivered by skin patches, it would vastly improve the availability of vaccines in the developing world, where inoculating with needles and syringes is often prohibitively expensive because of the costs of trained medical staff. In addition, skin patches do not need to be stored in refrigerators and they would solve the problem of re-using dirty needles and their safe disposal.
Professor Mark Prausnitz of the Georgia Institute of Technology in Atlanta, who led the research team, said the microneedle skin patch has the potential to revolutionise the way vaccines are delivered and administered because it is so simple and safe to use.
"The dissolving microneedle patch could open many new doors to immunisation programmes by eliminating the need for trained personnel to carry out the vaccination. This approach could make a significant impact because it could enable self-administration as well as simplifying vaccination programmes," Professor Prausnitz said.
"In this study, we have shown that a dissolving microneedle patch can vaccinate against influenza at least as well, and probably better than, a traditional hypodermic needle," he added.
Mice immunised with the skin patches shrugged off a subsequent infection with influenza virus a month after being vaccinated, just as well as mice inoculated with needles. However, when they were subjected to a second flu infection three months after vaccination, the mice with skin patches were better able to clear flu virus from their lungs compared to conventionally vaccinated mice, according to the study, which was published in the journal Nature Medicine.
The scientists believe this difference between the two groups of vaccinated animals is due to the inoculation occurring via the skin rather than the muscle. "The skin is a particularly attractive site for immunisation because it contains an abundance of the types of cells that are important in generating immune responses to vaccines," said Professor Richard Compans of Emory University School of Medicine, who collaborated on the study.
Ioanna Skountzou of Emory University said the vaccine molecules were freeze-dried and are stuck to the microneedles at room temperature using ultraviolet light. "Another advantage of these microneedles is that the vaccine is present as a dry formulation, which will enhance its stability during distribution and storage," Dr Skountzou said.
Sean Sullivan, of the Georgia Institute of Technology, added: "We envision people getting the patch in the mail or at a pharmacy and then self-administering it at home. Because the microneedles on the patch dissolve away into the skin, there would be no dangerous sharp needles left over."
The microneedles are just 650 microns in length – about six times the thickness of a human hair – and are thought to be too short to stimulate the skin's pain receptors while long enough to penetrate through the skin's outer protective layer.
Further trials in animals will still need to take place before the skin patches can be tried on human volunteers. It may still take many more years of clinical trials, however, before vaccination by skin patch becomes routine.
The plastic polymer that is used to form the microneedles of the skin patch, polyvinylpyrrolidone, is already being used in medical equipment placed inside the body with no serious side-effects. Scientists believe that vaccinating by skin patch could result in lower doses of vaccine being used because the skin is a more effective site of the body for inoculation.
Special immune cells in the skin are designed to capture foreign particles or the "antigens" present in vaccines, making vaccination by skin patch more efficient, the scientists said.
"The use of needles measuring just hundreds of microns in length not only eliminates pain and enables simple delivery through a thin patch, but also inherently targets antigens to the abundant antigen-presenting cells of the skin's epidermis and dermis," say the scientists in their study.
How the patch works
*Each skin patch is armed with about 100 microneedles that have been covered with molecules of the freeze-dried vaccine. When the needles penetrate the skin, they dissolve away within a few minutes, delivering the vaccine to the important cells of the body's immune system, which are always present in the skin.
*Scientists believe that when mass produced, the skin patches could work out to be no more expensive than conventional vaccines delivered by needles and syringes. However, the overall costs of a vaccination programme should be lower because skin patches would not require expensively-trained medical personnel, or the costs of disposing of hazardous waste in the form of dirty needles.
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